Update Dockerfile

Dockerfile

@@ -91,7 +91,9 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
 
 # ── Create ubuntu user ───────────────────────────────────────────────────────
-RUN useradd -m -s /bin/bash -u 1000 ubuntu \
+RUN if ! id -u ubuntu >/dev/null 2>&1; then \
+        useradd -m -s /bin/bash -u 1000 ubuntu; \
+    fi \
     && echo "ubuntu:ubuntu" | chpasswd \
     && echo "ubuntu ALL=(ALL) NOPASSWD:ALL" >> /etc/sudoers \
     && mkdir -p /tmp/runtime-ubuntu \